1. Field of the Invention
The present invention generally relates to instrumentation for logging conditions in down hole formations and, more particularly to transducer design and uses thereof in acoustic logging instrumentation. The acoustic transducer designs disclosed herein may be adapted to a variety of applications, including, for example, cement bond logging.
2. Background of the Invention and Description of the Prior Art
The use of piezoelectric materials such as a polycrystaline form of lead titanate or lead zirconate titanate, sometimes referred to as “PZT” material, has long been known. This material is generally formed as a continuous ceramic element, usually cylindrical in shape and sized to fit within the dimensions of a cement bond logging tool. Such tools are available in various diameters for use with well casings typically used in oil and gas drilling operations. Other shapes may be used in some logging tool instrumentation, depending upon the particular application. As is well known, these piezoelectric, ceramic PZT elements, according to their dimensions, have characteristic resonant frequencies. A given element or crystal may have more than one mode of vibration in which it behaves in accordance with its piezoelectric property. The crystal may vibrate in the presence of an electric field alternating at or near the resonant frequency of the crystal when imposed across the crystal. Or, the crystal may produce an alternating voltage in the presence of sufficient mechanical stress at or near the resonant frequency of the crystal. The ceramic crystal is usually coated on two opposing surfaces with an electrical conductor such as silver to which electrodes may be attached for connection to electronic circuitry. Thus, the crystal may be caused to vibrate at its resonant frequency by application of a suitable excitation signal applied to the two electrodes. Conversely, the crystal may vibrate in the presence of an acoustic disturbance or wave and generate an alternating electric voltage between the two electrodes.
In conventional cement bond logging tools, a cylindrical piezoelectric crystal having dimensions suited to behave in a predictable manner at a chosen frequency of resonance, may be used in both transmitting transducers and receiving transducers. In a transmitter, the crystal is excited by an electrical signal, for example 20 KHz, and caused to vibrate at that frequency, emitting acoustic energy into the surroundings—much as an antenna would. In a receiver, the crystal, tuned according to its dimensions to vibrate at the same 20 KHz frequency, produces an alternating 20 KHz voltage across its terminals when it is in the presence of the acoustic signal generated by the transmitter. The receiver thus acts as a sensor, responding to the acoustic energy that has traveled through the surroundings of the instrument containing the transducers.
The surroundings may include the well casing, the cement in the bore hole that surrounds the well casing, and the nearby rock formation through which the borehole has been drilled. The acoustic energy sensed by the receiving transducer is conveyed by the acoustic waveform as it is modified by attenuation, reflection, refraction and interference that may be encountered by the signal along the particular path between the transmitter and the receiver. Typically, the paths taken by the transmitted acoustic signal may include (a) the body of the logging tool; (b) the fluid in the well casing; (c) the well casing; (d) the cement or any other materials in the well outside of the well casing; and (e) the formation or lithography through which the well is bored. Operators of the surface instrumentation coupled to the logging tool sensor, by observing the received signal, may thus obtain information about the well casing, the cement bond and the surrounding lithography by recording and interpreting the received acoustic signal. The signal obtained from the receiver is typically processed to preserve essential information and formatted for display at the surface.
Operation of the cement bond logging tools must occur in some highly challenging environments. For example, the temperatures in a well may reach 350 to 400 degrees Fahrenheit. Further, the pressures in the well at depths on the order of many thousands of feet may reach 20,000 pounds per square inch (psi). These environmental conditions impose severe stresses upon the electronics and the mechanical structures of the instrument. Further, conventional PZT elements typically function most efficiently at or near resonance, to avoid the effects of spurious vibrations that can interfere with the intended signals, thus limiting their usefulness as transducers. For example, the crystals often generate harmonics of the desired frequency, which can result in intermodulation products within the band of interest near the fundamental or desired signal. In addition, operation of the crystal at only one frequency limits the utility of the transducers to the use of only the one frequency.
Other aspects of prior art logging tools that limit their utility and efficiency include the following. The processing of the data signals tends to be relatively slow in comparison to the amount of data that can be provided by the transducer. At the rates the tool is typically pulled toward the surface while taking the data, the data “packet” from each of the usual eight, radially (i.e., circumferentially) arranged sectors of the transducer must be transmitted to the surface with a delay imposed between the data for each sector. Thus, the data provided represents the conditions at a sequence of helically positioned sites along the outside of the well casing. Thus, no two adjoining data “packets” occur at the same depth or azimuth, and no complete sets of data “packets” provide information about the complete circumference, in all eight equally-spaced azimuth directions, of the well casing at a single depth. The result is that the data only represents a small and often insufficient sampling of the cement bond as the tool is pulled upward.
Moreover, because of the need to limit the upward rate, in feet-per-minute, that the tool is pulled toward the surface so that an adequate amount of data is obtained, substantial time is required to produce a complete cement bond log. Additional time is required just in setting up the instrument for operation. Especially time consuming is the time needed to calibrate the instrument for the particular logging operation. As is well known, the wire line used to support the logging tool in the well casing is a very long conductor having a large distributed reactive impedance characteristic in addition to the DC resistance of the conductor in the wire line. When the wire line is used to transmit analog signals to the surface instrumentation, this complex impedance and the length of the wire line must be compensated and usually involves tedious adjustments of the gain of each of the eight or more analog preamplifiers in the logging tool. Further, the particular adjustments are usually applicable only for the conditions existing at the time these adjustments are made because they are subject to being invalid as the logging tool is moved to another location, used with another surface vehicle, the temperature or other conditions in the well casing change, etc. In some logging tools, when a wire known as a “slickline” is used—simply a wire for supporting the tool that is not also used to transmit the data—the data in digital form may be stored in a memory device in or attached to the logging tool and accessed after the tool is returned to the surface.
The foregoing disadvantages and inefficiencies are substantially overcome by the features of the present invention to be described herein below.